Modern hard disc drives comprise one or more rigid discs that are coated with a magnetizable medium and mounted on the hub of a spindle motor for rotation at a constant high speed. Information is stored on the discs in a plurality of concentric circular tracks by an array of transducers ("heads") mounted to a controllably positionable actuator for radial movement relative to the discs.
Typically, such radial actuators employ a voice coil motor to position the heads with respect to the disc surfaces. The heads are mounted via flexures at the ends of a plurality of arms which project outward from an actuator body. The actuator body pivots about a cartridge bearing assembly mounted to the disc drive housing and normally disposed thereto at a position closely adjacent the outer extreme of the discs so that the heads move in a plane parallel with the surfaces of the discs.
The voice coil motor includes a coil mounted radially outward from the cartridge bearing assembly, the coil being immersed in the magnetic field of a magnetic circuit of the voice coil motor. The magnetic circuit comprises one or more permanent magnets and magnetically permeable pole pieces. When current is passed through the coil, an electromagnetic field is established which interacts with the magnetic field of the magnetic circuit so that the coil moves in accordance with the well-known Lorentz relationship. As the coil moves, the actuator body pivots about the pivot shaft and the heads move across the disc surfaces.
A closed loop digital servo system such as disclosed in U.S. Pat. No. 5,262,907 issued Nov. 16, 1993 to Duffy et al., assigned to the assignee of the present invention, is typically utilized to maintain the position of the heads with respect to the tracks. Such a servo system obtains head position information from servo blocks written to the tracks during disc drive manufacturing to maintain a selected head over an associated track during a track following mode of operation. A seek mode of operation, which comprises the initial acceleration of a head away from an initial track and the subsequent deceleration of the head towards a destination track, is also controlled by the servo system. Such seek operations are typically velocity-controlled, in that the velocity of the head is repetitively measured and compared to a velocity profile, with the current applied to the coil being generally proportional to the difference between the actual and profile velocities as the head is moved toward the destination track.
It will be recognized that a continuing trend in the industry is to provide disc drives with ever increasing data storage and transfer capabilities, which in turn has led to efforts to minimize the overall time required to perform a disc drive seek operation. A typical seek operation includes an initial overhead time during which the disc drive services its own internal operations, seek time during which the head is moved to and settled on the destination track, and latency time during which the drive waits until a particular sector on the destination track reaches the head as the discs rotate relative to the heads.
Seek times have typically been minimized through the application of relatively large amounts of current to the coil during the acceleration and deceleration phases of a seek operation. Latency times have also been continually minimized through continued increases in the rotational speeds of the discs (which in some disc drives have reached 10,000 revolutions per minute).
A problem resulting from these improvements in disc drive seek performance, however, is the increase in the amount of heat that is generated within the drive. Particularly, the spindle motor used to rotate the discs is typically one of the largest sources of heat in the drive, and the amount of heat dissipated within the drive generally increases with increases in rotational speed of the discs and the amount of drag upon the discs. Moreover, after repetitive seeking, resistive power losses in the actuator coil of the voice coil motor tend to also generate significant amounts of heat within the drive. Because the coil is mechanically isolated from the magnetic circuit of the voice coil motor, over time the temperature of the coil can exceed that of the rest of the disc drive by several degrees, creating a localized "hot spot" within the disc drive.
It is well known that the amount of current that can be passed through an actuator coil is generally a function of the direct current (dc) resistance of the coil, which in turn generally increases proportionally with the temperature of the coil. Hence, as coil temperature increases, so does the resistance; thus, over time lesser amounts of current can be applied to the coil, which in turn generally increases the time required to move a selected head from an initial track to a destination track during a seek operation. Moreover, elevated voice coil motor temperatures with respect to ambient can further adversely affect the ability of the disc drive to achieve optimal levels of performance, as the field strength of the magnetic circuit of a voice coil motor is generally a function of temperature and generally weakens as temperature is increased. Thus, the localized heating of the magnetic circuit further tends to increase the seek time of a disc drive.
Additionally, elevated voice coil motor temperatures can result in the degradation of adhesive and insulative materials used in the construction of the voice coil motor. Such degradation can lead to internal contamination of the disc drive as well as to the shorting of the coil.
Accordingly, there is a continual need for improvements in the art whereby data transfer performance of a disc drive can be increased while accommodating problems associated with the generation of heat within the drive as higher levels of drive performance are obtained.